The Prostate, Lung, Colorectal and Ovarian (PLCO) Cancer Screening Trial is a large, randomized study to determine whether the use of certain screening tests will reduce the risk of dying of those four cancers. In addition to answering questions about the screening tests, the PLCO asked questions about many aspects of the study participants? health and collected biospecimens (blood and some tissues) to answer many other questions about cancer. Ten screening centers located across the United States enrolled nearly 155,000 people ages 55 to 74 with no previous history of prostate, lung, colorectal, or ovarian cancer. These participants were randomized to either an intervention arm (an organized screening program within the trial) or control arm (which received the usual care from their healthcare providers and which sometimes included screening.) PLCO centers began enrolling participants in 1993 and completed enrollment in 2001. Trial participants are unpaid volunteers recruited from the general population in the geographic area of each of the screening centers; there were approximately 78,000 women and 76,000 men enrolled in the study.